Corrosion resistant nickel-molybdenum alloys



United CORROSION RESISTANT NICKEL-MOLYBDENUM ALLOYS George Norman Flint,Sheldon, Birmingham, England, assignor to The International NickelCompany, Inc., New York, N.Y., a corporation of Delaware The presentinvention relates to corrosion resistant alloys and, more particularly,to nickel-molybdenum alloys characterized by a high degree of resistanceto corrosive media such as hydrochloric, sulphuric phosphoric and otheracids.

It is well known that nickel-molybdenum-alloys are widely used for themanufacture of chemical plant and equipment which is required towithstand corrsive attack by hydrochloric, sulphuric, phosphoric andother acids. Broadly, the alloys contain from 10 to 40% molybdenum andfrom 2 to 25% iron, the balance being nickel and small amounts ofcarbon, silicon and manganese. Alloys of this kind at present on themarket contain from 26 to 30% molybdenum, from 4 to 7% iron, from 0.1 to1.5% silicon and from 0.1 to 1.0% manganese, the balance being nickeland impurities.

As will be readily recognized by those skilled in the art, these alloysfrequently have to be fabricated by welding. The fabrication operationentails localized heating of the parts to be joined to the temperatureof fusion, and has the unfortunate result that the material in a zoneadjacent or nearly adjacent to the fused zone is rendered susceptible tointergranular corrosion in some corrosive media, especially in hot,strong hydrochloric acid, so that the equipment becomes useless for thepurpose for which it was intended.

A review of the literature, e.g., U.S. Patents Nos. 1,375,082 and1,375,083, indicates that alloys containing about 80% to 90% nickel, 10%molybdenum and up to 10% iron were considered useful for the purpose ofresisting the corrosive attack of acids. According to the disclosure setforth in U.S. Patent No. 1,710,445, the corrosion resistance could beenhanced by broadening the molybdenum content to about 15% to 40% of thealloy. The latter alloy optionally included up to 4% manganese, up toabout 1.5% silicon for purposes of fluidity, about 0.3% vanadium as ascavenger and up to about 0.5% carbon. Such alloys were apparentlyconsidered unsatisfactory because they required a heat treatingoperation as indicated in U.S. Patent No. 1,836,317. In this latterpatent it was proposed that the necessity of employing a heat treatingoperation was obviated if the carbon content of the alloys was keptbelow 0.6%, e.g., less than 0.2%. However, the patentee indicated thatthe alloys were susceptible only to a moderate amount of working, e.g.,forging or rolling.

The prior art apparently still considered such alloys unsatisfactory.For example, in U.S. Patent No. 2,109,285 it is disclosed that thesilicon and carbon contents usually employed lessened the corrosionresistance and that the contents thereof should be kept below 0.10% and0.05%, respectively. It was considered that iron, chromium, manganese,copper and vanadium did not enhance corrosion resistance. In U.S. PatentNo. 2,196,699 it was proposed to incorporate 0.08% to 6% antimony in theprior art alloys for the purpose of increasing the corrosion resistanceproperties thereof. After a period of twenty years and many proposals,the art was still confronted with the problem of commercially providing.

a satisfactory alloy capable of exhibiting the desired retates Patentice si'stance to corrosive effects of acids in heat affected zones.

As mentioned hereinabove, the metallurgist now appreciates that thecorrosion resistance of such alloys was' seriously impaired because ofdeleterious intergranular corrosion resulting from fabricationoperations.' As far as applicant is aware, the only known way ofovercoming this problem is described in U.S. Patent No. 2,237,872, andconsists of a special heat treating process comprising heat treating thealloys for upwards of 2 hours but not exceeding 72 hours within therange of 900 C. and 1175 C. Such heat treatment is, however, difficultto apply in practice owing to the practical problems involved in heattreating large fabricated pieces of equipment and the distortiondeveloped during heat treatment. In fact, as referred to hereinabovewith respect to U.S. Patent No. 1,836,317, it is commercially desirableto avoid heat treating operations which must be applied to thefabricated commercial equipment to be used for handling hot, strongacids such as hydrochloric acid.

In addition to the problem of intergranular corrosion describedhereinbefore, other problems have confronted the art such as thoseinvolved in but working proposed alloys prior to the fabrication ofequipment therefrom. Many alloys were so susceptible to cracking duringhot working as to be useless for practical, commercial use in wroughtforms.

Thus, hitherto, there still existed the demand for and need of an alloycharacterized not only by a high degree of resistance in corrosive mediasuch as hydrochloric acid, sulphuric acid, etc., but which (1) could behot worked without the incurrence of cracking, (2) could be easilyfabricated, as by welding, such that when subjected to the action ofcorrosive acids the alloy would not undergo the deleteriousintergranular corrosion in heat affected zones which characterizedfabricated equipment made from alloys of the prior art, and (3) did notrequire cumbersome, tedious and expensive heat treatments to conditionfabricated equipment made of the alloy against the intergranularcorrosive attack described hereinabove.

Although attempts were made to overcome the foregoing difficulties andother difiiculties, none, as far as I am aware, was entirely successfulwhen carried into practice commercially on an industrial scale.

It has now been discovered that nickel-molybdenum alloys containingspecial amounts of vanadium are highly resistant to corrosive media suchas hydrochloric acid, sulphuric acid, phosphoric acid and other acids,are hot workable, are easily fabricated as by welding, and do notrequire heat treatment subsequent to fabrication to minimizeintergranular corrosive attack in heat affected zones when subjected toa corrosive environment.

It is an object of the present invention to provide nickelmolybdenumalloys which are resistant to the influence of corrosive media such ashydrochloric acid, sulphuric acid, phosphoric acid, etc.

Another object of the invention is to provide corrosionresistantnickel-molybdenum alloys which can be hot worked, e.g., byforging orrolling.

The invention also contemplates providing nickelmolybdenum alloys whichdo not undergo deleterious intergranular corrosion in heat affectedzones when used as fabricated chemical processing equipment andsubjected to attack of corrosive media such as hot hydrochloric acid.

It is a further object of the invention to provide nickelmolybdenumalloys which do not require heat treatment to resist intergranularcorrosive attack in heat affected zones when used as chemical processingequipment;

The invention further contemplates providing hot workable,nickel-molybdenum alloys capable of withstandtack of corrosive mediasuch as hydrochloric acid, sul- Patented Nov. 8, 1960 r phuric acid,phosphori'c acid and other acids without requiring heat treatment tominimize intergranular corrosive attack.

Other objects and advantages will become apparent from the followingdescription.

Generally speaking, the present invention provides nickel-molybdenumalloys containing special amounts of vanadium and which are highlyresistant to corrosive media such as hydrochloric acid, sulphuric acid,phosphoric acid and other acids, are hot workable, and do not undergodeleterious intergranular corrosion in heat af-' fected zoneswhen'usedas fabricated structures in chemical processing equipment. These alloyscontain by Weight about 20% to about 35% molybdenum, upi to about 15%iron, vanadium in an amount of 1.2% to-2.3%,. the balance beingessentially nickel. tent shouldbe not less than 1.1% and particularlygood results are obtained concerning both corrosion resistance and hotworkabilityiin accordance with the invention with alloys containing 26%to 30% molybdenum, up: to 7% iron, vanadium in an amount of 1.2% to2.3%, e.g., 1.8% to 2.1%, the balance beingnickel.

The molybdenum content of these vanadium-containing alloys according tothe invention is between 20 and 35%. With less than 20% molybdenum thegeneral corrosion resistance of the alloy is very poor. As themolybdenum content increases the corrosion resistance increases,

but so does thedifficulty of hot-working the alloys, and

the upper limit of 35% is imposed by the extreme difficulty of forgingthe alloys with higher molybdenum contents with an economic yield. Thebest combination of corrosion resistance and forgeability is obtainedwith from 26 to 30% molybdenum.

Chromium has an adverse effect on forgeability, so the alloys arepreferably chromium-free, although a little chromium, i.e., up to 5%,can be tolerated.

Some silicon is commonly present, as it is added as 'a deoxidiser, butsilicon also has an adverse effect on forgeability, and the siliconcontent should'not exceed 1.0%. Preferably it does not exceed 0.8%.

If each ofthe elements molybdenum, vanadium, silicon and chromium ispresent simultaneously in anamount at or close to the maximum valuegiven above, the alloy may still be virtually unforgeable. It istherefore necessary to impose a further limitation and specifically toensure that the value of the expression does not exceed 30.

Some of the molybdenum may be replaced by an equal percentage oftungsten, since'tungsten is beneficial in increasing the resistance tocorrosion after welding. However, tungsten renders the alloy morediflicult to forge, and it should not amount to more than 10% of thealloy.

When tungsten is present the expression defining the overall limitationin the composition of the alloy is modified, and in fact the expressionmust not exceed 30. Subject to the limitations expressed above thevanadium content should be as high as possible.

The vanadium con- The iron content of the alloys should be low, as thecorrosion resistance falls with increasing amounts of iron in thealloys. It is convenient to use ferro-alloys in making the alloys of theinvention, -so iron is commonly present, but for best results thereshould not be more than 7% iron. However, good corrosion resistance isobtained with 10% and reasonably good resistance up to a maximum of 15%iron.

Cobalt may be present as an impurity in the nickel, and larger amountsup to-5% may replace part of the nickel.

The carbon content is preferably as low as possible since the corrosionresistance falls off rapidly with increasing carbon content. It shouldnot exceed 0.15%, though 0.25% may be tolerated if a lower level ofcorrosion resistance is acceptable, that is to say, if the alloy is notto be subjected in use to highly corrosive attack.

Manganese is another elementcommonly used as a deoxidizer and thereforegenerally present. The manganese content may be up to 3%, but ispreferably from 0.1 to 0.5%.

Aluminium may be used as a deoxidiser and may then be presentin theresultant alloy in amounts of 0.1 to 0.2%. It may be present up to 2.0%without harm.

Traces of other elements, such as calcium, added to deoxidise andimprovethe workability of the alloys may be'present. Copper, which has a verydeleterious effect upon workability, should so far as possible beabsent, but can be tolerated up to 0.5%.

The balance of the alloy except for impurities and residual deoxidisersis nickel.

I have also found that the resistance to intergranular corrosionproduced by the vanadium increases with increase in the molybdenumcontent, and to some extent also with increase in the silicon content.By using molybdenum contents in the upper part of the preferred range,therefore, we can obtain substantially the optimum results with ratherless vanadium than would otherwise be required.

The improvement brought about by means of the invention is clearly shownby the results obtained with alloys treated to simulate the efiectofwelding. In this treatment one end of a forged round bar of the alloy4 inches long by /2 inch diameter was fused in an oxyacetylene flame,the bar being exposed to the flame for about 15 seconds and the metalfused for about 3 seconds, and then air-cooled. A test piece /2 inchlong and inch in diameter was then machined from the heated end,immersed in a boiling 10% hydrochloric acid solution and then sectionedlongitudinally. Some specimens were immersed for 28 days and others for61 days. The depth of penetration of the corrosive attack was measuredfrom each side, giving two values for each specimen. In all the alloystested the rate of corrosion under similar conditions in regions notaffected by the heat is less than 0.001 inch in 28 days.

Some of the alloys thus treated and tested were free from or containedtoo little vanadium. These are identified by the letters A, B, and C.Others were in accordance with the invention and are identified bynumbers. The compositions of the alloys and the results obtained were asfollows:

Composition-Percent Depth of Corrosion (Inches) After- 28 days Nil N11soot-f The beneficial effect of increase in the molybdenum content isshown by comparison of alloys Nos. 3, 4 and 5, and that of silicon bycomparison of alloys Nos. 2 and 3, particularly by the figures ofcorrosion after 61 days.

The fabricated and welded plant or equipment made from our alloys is notheat-treated after being welded, since the vanadium content renders thisunnecessary. Thus such plant or equipment made from welded alloys isused in the as-welded state in contact with hydrochloric acid or othercorrosive medium known normally to produce intergranular corrosion inthe absence of heat-treatment.

The alloys may, however, advantageously be subjected to acarbide-coarsening heat-treatment before being welded in order to renderthe molybdenum carbide less readily taken into solution on thesubsequent welding operation. Such a heat treatment may comprise heatingthe alloy to a temperature above 1175 C. and below the solidustemperature (approximately 1315 C.), cooling to a temperature within therange 1150 C. to 900 C. and maintaining within this temperature rangelong enough to allow the carbon in solution to reprecipitate on existingcarbide particles. The alloy should be reasonably quickly cooled fromthe latter temperature e.g., by air cooling.

For example the alloy may be heated at 1250 C. for 20 minutes, cooled inthe furnace to 1000 C. and aircooled to room temperature. Alternatively,the alloy after heating at 1250 C. may be rapidly cooled to atemperature within the range 1150 C. to 900 C. and maintained at thistemperature for a period of time e.g., 1 hour, lon enough to coarsen thecarbides and then cooling, e.g., in air or water.

A further alternative consists in oscillating the temperature of thefurnace in which the alloy is being treated to values above 1175 C. andthen to values within the range 1150" C. to 900 C. Finally the alloy isair cooled or otherwise quickly cooled from the latter temperature.

On the other hand any heat treatment serving to precipitate carbides ina finely dispersed state, e.g., heating to 1200 C. followed byair-cooling, should be avoided.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. A corrosion-resistant, hot-workable alloy suitable for use inchemical plant equipment, said alloy being further characterized in thatit does not undergo deleterious intergranular corrosion in heat affectedzones when subjected in the as-welded condition to attack by corrosivemedia, said alloy consisting essentially of 26% to 30% molybdenum, up toiron, 1.8% to 2.1% vanadium, up to 0.8% silicon, 0.01% to 3% manganese,up to 0.25 carbon, up to 5% cobalt, and the balance essentially nickel.

2. A new and improved nickel-base alloy characterized by the propertiesof corrosion resistance, hot workability and resistance to deleteriousintergranular corrosion in heat affected zones when used in theas-welded condition and subjected to attack by corrosive media, saidalloy consisting essentially of 26% to 30% molybdenum, up to iron notless than 1.1% and-up to 2.3% vanadium, up to 1% silicon, up to 3%manganese, up to 0.25% carbon, and the balance essentially nickel.

3. A corrosion-resistant, hot-workable alloy suitable for use inchemical plant equipment, said alloy being further characterized in thatit does not undergo dele-' terious intergranular corrosion in heataffected Zones 6 when subjected in the as-welded condition to attack bycorrosive media, said alloy consisting essentially of about 20% to about35% molybdenum, up to about 15% iron,

1.2% to 2.3% vanadium, up to 1% silicon, up to 3% manganese, up to 0.25%carbon, up to 5% cobalt, up to 5% chromium, up to 2% aluminum, andthebalance essentially nickel, the molybdenum, vanadium, silicon andchromium being present in such amounts that the relationship expressedby the following formula is satisfied:

equals a numerical value not exceeding 30.

4. A corrosion-resistant, hot-workable nickel-base alloy suitable foruse in chemical plant equipment and being further characterized in thatit resists intergranular corrosion in heat affected zones when subjectedin the aswelded condition to attack by corrosive media, said alloyconsisting essentially of about 20% to about 35% molybdenum, up to about15% iron, 1.2% to 2.3% vanadium, up to 1% silicon, up to 3% manganese,up

to 0.2% carbon, up to 5% cobalt, and the balance essentially nickel.

5. A corrosion-resistant, hot-workable alloy suitable for use inchemical plant equipment, said alloy being further characterized in thatit does not undergo deleterious intergranular corrosion in heat affectedzones when subjected in the as-weled condition to attack by corrosivemedia, said alloy consisting essentially of about 20% to about 35%molybdenum plus tungsten, the amount of tungsten not exceeding 10%, upto about 15% iron, 1.2% to 2.3% vanadium, up to 1% silicon,

up to 3% manganese, up to 0.25% carbon, up to 5% cobalt, up to 5%chromium, up to 2% aluminum, and

the balance essentially nickel, the molybdenum, tungsten, vanadium,silicon and chromium being present in such amounts that the relationshipexpressed by the following formula is satisfied:

equals a numerical value not exceeding 30.

6. A method of improving the resistance to intergranular corrosion of analloy consisting essentially of about 26% to about 30% molybdenum, up to7% iron,

1.2% to 2.3% vanadium, up to 0.8% silicon, up to 0.5%

manganese, up to 0.15% carbon and the balance essentially nickel, whichcomprises heating said alloy at a temperature above about 1175 C. andbelow the solidus temperature of said alloy, cooling to a temperature ofbetween about 1150 C. to about 900 C. and maintaining this temperaturecondition for a period sufficient for the carbon in solution tore-precipitate on existing carbide particles, and then further coolingsaid alloy.

pub. by Engineering Alloys Digest, Inc., Upper Montv clair, NJ.

Chemical Engineering Progress, vol. 48, August 1952, pages 377-380.

ORNL-2181, October 18, 1956. Final report on the Development and Testingof Vacuum Melted Nickel- Molybdenum Alloys with Minor AlloyingAdditions," 7

Preston et al.

1. A CORROSION-RESISTANT, HOT-WORKABLE ALLOY SUITABLE FOR USE INCHEMICAL PLANT EQUIPMENT, SAID ALLOY BEING FURTHER CHARACTERIZED IN THATIT DOES NOT UNDERGO DELETERIOUS INTERGRANULAR CORROSION IN HEAT AFFECTEDZONES WHEN SUBJECTED IN THE AS-WELDED CONDITION TO ATTACK BY CORROSIVEMEDIA, SAID ALLOY CONSISTING ESSENTIALLY OF 26% TO 30% MOLYBDENUM, UP TO10% IRON, 1.8% TO 2.1% VANADIUM, UP TO 0.8% SILICON, 0.01% TO 3%MANGANESE, UP TO 0.25% CARBON, UP TO 5% COBALT, AND THE BALANCEESSENTIALLY NICKEL.